Method of forming a flexible circuit board

ABSTRACT

A process of forming a flexible circuit board for ink jetting is provided. The process includes the steps of: providing an insulation tape; forming conductive traces on the insulation tape; and forming a photo-polymer layer filling between the conductive traces, wherein parts of the conductive traces are exposed to form a plurality of contacts. The material of the insulation tape can be polyimide, Teflon, polyamide, polymethylmethacrylate, polycarbonate, polyester, polyamide polyethylene-terephthalate copolymer, or any combination of the above materials. The material of the photo-polymer layer can be solder mask or polyimide.

[0001] This application is a continuation of application Ser. No.10/128,354, filed Apr. 24, 2002, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates in general to a method of forming aflexible circuit board, and more particularly to a method of fabricatinga flexible circuit board by using photo-polymer.

[0004] 2. Description of the Related Art

[0005] Flexible circuit boards used in the cartridge of an ink-jetprinter serve as medium to lead the driving current to the chip for inkjetting. The driving current drives the cartridge and enables thecartridge to jet ink.

[0006]FIG. 1 shows a conventional flexible circuit board and itsrelative position to the dimple of the printer. Polyimide (PI) is acommonly used substrate 104 for the conventional flexible circuit board.Copper (Cu) and gold (Au) are two widely used materials for theconductive traces 106 in the flexible circuit board. The dimples 110 ofthe printer circuit contact the conductive traces 106 through holes 108formed by tape automated bonding (TAB).

[0007] Etching and punching are two typical TAB manufacturing processes.Etching process is characterized by etching the tape while the punchingprocess is characterized by punching the tape to form the holes.

[0008]FIG. 2A˜FIG. 2J illustrate the conventional etching process. Onthe substrate 202, such as polyimide (PI), a copper film 204 with athickness of about 100 Å is formed by sputtering. On the bottom side ofthe substrate 202 and over the copper film 204, photo-resistors (PR) 206are formed. After exposing and developing the PRs 206, the patterns ofthe holes and the conductive traces are defined. Next, as shown in FIG.2F, on the front side of substrate 202 with the exposed copper film 204,a copper layer 208 with several μm is plated. Then, as shown is FIG. 2Gthe substrate 202 is etched to form the holes 210 at the bottom side ofsubstrate 202. Then, the photo-resistors at both sides of substrate 202are then removed, and the configuration is presented FIG. 2H. Thefollowing step as shown in FIG. 2I is a photolithography process,including steps of forming a photo-resister layer, exposing, developingand etching, to remove parts of the copper film 204 that are notprotected by the copper layer 208. Finally, as shown in FIG. 2J, aninsulation layer 212 is formed over the copper layer 208 for the purposeof protection and electrical insulation. Noted that the configuration ofthe holes 210 and the copper layer 208 are performed at the oppositesides of the substrate 202.

[0009] The conventional etching process has the following drawbacks:time consuming, producing thick and sticky precipitate and large amountof wastewater, high cost and low yield rate.

[0010]FIG. 3A to FIG. 3I show the conventional punching method to formholes on an insulation layer.

[0011] As shown in FIG. 3A and FIG. 3B, an adhesive layer 304 is coatedon the substrate 302. Then, the substrate 302 coated with the adhesivelayer 304 is punched to form holes 306. Next, a copper layer 308 isadhered over the substrate 302 coated with the adhesive layer 304. Then,as shown in FIG. 3E to FIG. 3H, a photo-resistor layer 310 is formed onthe copper layer 308. After the photolithography process, includingexposing, developing and etching, the pattern of the copper layer 308 isdefined. Finally, as shown in FIG. 31, an insulation adhesive layer 312is formed on one side of the copper layer for the purpose of protection.

[0012] Compared with the etching process as mentioned before, thispunching process is shorter in procedure, no problem of wastewater andlower cost. However, the intervals between each two holes are large andhard to reduce. So that, less holes can be formed in the same area,which therefore influences the precision contact between the printer andthe TAB. Further more, the punching step could easily cause the breakageof the substrate and thus reduce the yield and increase the cost.

SUMMARY OF THE INVENTION

[0013] It is therefore an object of the invention to provide a method offabricating flexible circuit board without having the problems ofproducing contaminating developer but with the advantages of shorterprocedure, lower cost, high hole resolution, and high yield rate, whichis suitable for mass production.

[0014] An improved and simplified process of forming a flexible circuitboard for ink jetting comprises the steps of: providing an insulationtape; forming conductive traces on the insulation tape; and forming aphoto-polymer layer filling between the conductive traces, wherein partsof the conductive traces are exposed to form a plurality of contacts.The material of the insulation tape can be polyimide, Teflon, polyamide,polymethylmethacrylate, polycarbonate, polyester, polyamidepolyethylene-terephthalate copolymer, or any combination of the abovematerials. The material of the photo-polymer layer can be solder mask orpolyimide.

[0015] It is another object of the invention to provide a flexiblecircuit board for ink jetting, comprising: an insulation tape as asubstrate; a plurality of conductive traces on the insulation tape; anda photo-polymer layer filling between the conductive traces, whereinparts of the conductive traces are exposed to form a plurality ofcontacts.

[0016] Other objects, features, and advantages of the invention willbecome apparent from the following detailed description of the preferredbut non-limiting embodiments. The following description is made withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 (Prior Art) shows a conventional flexible circuit board andits relative position to the dimple of the printer.

[0018]FIG. 2A˜FIG. 2J (Prior Art) illustrate the conventional etchingprocess.

[0019]FIG. 3A to FIG. 3I (Prior Art) show the conventional punchingmethod to form holes on an insulation layer.

[0020]FIG. 4A to FIG. 4J show a portion of the process of manufacturingthe flexible circuit board according to a preferred embodiment of theinvention.

[0021]FIG. 5 shows the process of forming, exposing, developing andpost-curing the photo-polymer layer.

[0022]FIG. 6 shows the flexible circuit board of the invention and itsrelative position to the dimple 610 in the printer.

[0023]FIG. 7 shows the flexible circuit board of the invention and thechip.

[0024]FIG. 8 (Prior Art) shows the conventional flexible circuit boardand the chip.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 4A to FIG. 4J show a process of manufacturing the flexiblecircuit board according to a preferred embodiment of the invention.First, as shown in FIG. 4A and FIG. 4B, an adhesive layer 404 is coatedover the insulation tape 402 made of the material such as polyimide(PI). And then, a conductive layer 406, the material of which is such ascopper or gold, is formed over the adhesive layer 404. In the practicalapplication, the conductive layer 406 could be formed by adhering acopper foil or a gold foil on the insulation tape 402. The preferredthickness of the copper foil or the gold foil is at the range of about10 μm to 50 μm.

[0026] Next, as shown in FIG. 4C to FIG. 4G, a photo-resistor layer 408is formed over the conductive layer 406. After defining the conductivelayer 406 to a desired pattern, the photo-resistor layer 408 is removed.By now, the insulation tape 402 has conductive traces with a desiredpattern on it.

[0027] Next, as shown in FIG. 4H, a photo-polymer layer 410 is formedover the insulation tape 402 with conductive traces 406. Preferably,formation of the photo-polymer layer 410 can be performed by one ofscreen printing, spray coating, curtain coating and roller coatingtechniques. The preferred material of the photo-polymer layer 410 can besolder mask. After exposed to the light, solder mask could becrosslinked by radiation. Crosslinked solder mask is strong in structureand insoluble in developer (developing solution), acting just as anegative photo-resistor. Moreover, PR materials having the similarphysical and chemical characteristics such as polyimide (PI) and thelike are also applicable herein.

[0028] As shown in FIG. 4I and FIG. 4J, a portion of the photo-polymerlayer 410 is removed to expose parts of the conductive traces 406 bydeveloping. After a post-curing process, the photo-polymer layer 410 ishardened.

[0029]FIG. 5 shows the process of forming, exposing, developing andpost-curing the photo-polymer layer 410. PSR9000 series A01 typephoto-solder mask (available from Taiwan Taiko Ink Corp.) is selected asthe preferred material of the photo-polymer layer 410 in the embodimentof the invention. The preferred ratio of the main agent and the hardeneris about 7:3. After the photo-polymer layer 410 is coated (step 502), anexposing step 504 is performed. The ideal exposure energy is about280˜420 mJ/cm². During the developing process 506, 1 wt % Na₂CO₃solution is used as the developer, and the photo-polymer 410 isdeveloped at the temperature lower than 30 ? for about 60˜90 sec. Atpost-curing 508 step, the flexible circuit board is placed at a hot airconvention oven at 150 ? for about 50 min.

[0030] Besides polyimide (PI), the material of the insulation tape canbe other polymer films such as Teflon, polyamide,polymethylmethacrylate, polycarbonate, polyester, polyamidepolyethylene-terephthalate copolymer, or any combination of the abovematerials.

[0031] Thus, the flexible circuit board fabricated according to thepreferred embodiment of the invention has an inventive structure asdescribe below. Unlike the prior disclosure, the conductive traces 406and the photo-polymer layer 410 are formed on the same side of theinsulation tape 402; also, the photo-polymer layer 410 fills between theconductive traces 406, not completely covering the conductive traces406. The photo-polymer layer 410 thus forms the holes 420 exposing partsof the conductive traces 406 to form a plurality of contacts (see FIG.4J). In other words, the photo-polymer layer 410 has the holes 420formed therein so that parts of the conductive traces 406 are notcovered by the photo-polymer layer 410 to form a plurality of uncoveredcontacts that are exposed by the holes 420.

[0032] Referring to FIG. 1, the conventional dimple 110 of the printer(not shown) contacts with the conductive layer 106 of the flexiblecircuit board at the other side of the insulation tape 104 through thehole 108. However, the conductive layer 406 of the flexible circuitboard of the invention is positioned at the side of the insulation tape402 near the dimple 610 of the printer. Therefore, it apparently showsthat the dimple 610 of the printer contacts the conductive layer 406more directly and precisely.

[0033] Moreover, the flexible circuit board fabricated according to theinvention has the practical advantage of less opportunity of shortcircuit. Referring to FIG. 7, after the flexible circuit board of theinvention adheres to the chip 702, the conductive layer 406 slightlybends because the insulation tape 402 and the chip 702 are at the sameside of the conductive layer 406. On the contrary, after theconventional flexible circuit board adheres to the chip 802 as shown inFIG. 8, the conductive layer 208 bends sharply because the insulationtape 202 and the chip 802 are at the opposite sides of the conductivelayer 208. With the character of less bending of the conductive layer406, the flexible circuit board of the invention has the advantage ofless opportunity of short circuit.

[0034] Summarily, forming the contacts on the insulation tape accordingto the preferred embodiment of the invention has the followingadvantages and inventive features:

[0035] (1) Photo-polymer applied in the invention provides higherconductive traces density and better coverage than the conventional dryfilm.

[0036] (2) Solution with weak alkalinity is used as the developer, whichis low in cost in terms of reagent and equipments, and causes noenvironmental pollution.

[0037] (3) The conductive layer hardly bends so that the problem ofshort circuit is eliminated.

[0038] (4) The configuration of the conductive traces, the photo-polymerlayer and the holes are performed at the same sides of the insulationtape.

[0039] (5) The photo-polymer layer fills between the conductive traces,thereby exposing parts of the conductive traces 406 to form a pluralityof holes.

[0040] Furthermore, the process of the invention has no require forperforming the step of punching holes on the insulation tape; thus,overall manufacturing time is shortened, production cost is decreased,hole resolution is good and production yield is improved (up to 99%).Accordingly, the method provided by the invention is suitable for massproduction.

[0041] While the invention has been described by way of an example ofmanufacturing a flexible circuit board (FCB), it is to be understood TABdevice is also within the scope of the invention since FCB is commonlybounded with the chip through a TAB process

[0042] While the invention has been described by way of example and interms of a preferred embodiment, it is to be understood that theinvention is not limited thereto. On the contrary, it is intended tocover various modifications and similar arrangements and procedures, andthe scope of the appended claims therefore should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar arrangements and procedures.

What is claimed is:
 1. A method of forming a flexible circuit board forink jetting, comprising of: providing an insulation tape; formingconductive traces on the insulation tape; and after the conductivetraces are formed on the insulation tape, forming a photo-polymer layerthat fills between the conductive traces, wherein the photo-polymerlayer has holes formed therein so that parts of the conductive tracesare not covered by the photo-polymer layer to form a plurality ofuncovered contacts that are exposed by the holes.
 2. The methodaccording to claim 1, wherein the insulation tape comprises a polymerfilm.
 3. The method according to claim 2, wherein a material of thepolymer film comprises at least one of polyimide (PI), Teflon,polyamide, polymethyl methacrylate, polycarbonate, polyester, andpolyamide polyethylene-terephthalate copolymer.
 4. The method accordingto claim 2, wherein a material of the polymer film is polyimide (PI). 5.The method according to claim 1, wherein the forming the conductivetraces comprises of: forming an adhesive layer on the insulation tape;adhering a conductive layer on the adhesive layer; and defining thepattern of the conductive layer to form the conductive traces.
 6. Themethod according to claim 5, wherein a material of the conductive layeris copper.
 7. The method according to claim 5, wherein a material of theconductive layer is gold.
 8. The method according to claim 5, wherein athickness of the conductive layer is about 10 μm to 50 μm.
 9. The methodaccording to claim 1, wherein the photo-polymer layer is formed bycoating.
 10. The method according to claim 1, wherein the photo-polymerlayer is formed by one of screen printing, spray coating, curtaincoating and roller coating.
 11. The method according to claim 1, whereinthe photo-polymer layer is made of solder mask.
 12. The method accordingto claim 1, wherein the photo-polymer layer is made of polyimide. 13.The method according to claim 1, wherein the forming a photo-polymerlayer comprises coating, exposing, developing and post-curing.
 14. Themethod according to claim 13, wherein the photo-polymer layer is made ofsolder mask and the solder mask comprises a main agent and a hardenerwith a ratio of about 7:3.
 15. The method according claim 13, whereinthe exposing is performed with an exposure energy of about 280˜420mJ/cm².
 16. The method according to claim 13, wherein the post-curing isperformed at a hot air convention oven at about 150 ? for about 50 min.